1. Field of the Invention
The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to an alkali metal cell suitable for current pulse discharge applications with reduced or no appreciable voltage delay. These benefits are realized by the provision of a sulfate additive in the electrolyte.
2. Prior Art
The initial drop in cell potential during the application of a short duration pulse is termed voltage delay, and reflects the resistance of the cell, i.e., the resistance due to the cathode, the cathode-electrolyte interphase, the anode, and the anode-electrolyte interphase. In the absence of voltage delay, the resistance due to passivated films on the anode and/or cathode is negligible. However, the formation of a surface film is unavoidable for alkali metal, and in particular, lithium metal anodes, and for lithium intercalated carbon anodes, due to their relatively low potential and high reactivity towards organic electrolytes. Thus, the ideal anode surface film should be electrically insulating and tonically conducting. While most alkali metal, and in particular, lithium electrochemical systems meet the first requirement, the second requirement is difficult to achieve. In the event of voltage delay, the resistance of these films is not negligible, and as a result, impedance builds up inside the cell due to this surface layer formation which often results in reduced discharge voltage and reduced cell capacity. In other words, the drop in potential between the background voltage and the lowest voltage under pulse discharge conditions, excluding voltage delay, is an indication of the conductivity of the cell, i.e., the conductivity of the cathode, anode, electrolyte, and surface films, while the gradual decrease in cell potential during the application of the pulse train is due to the polarization of the electrodes and the electrolyte.
Thus, the existence of voltage delay is an undesirable characteristic of alkali metal/mixed metal oxide cells subjected to current pulse discharge conditions in terms of its influence on devices such as medical devices including implantable pacemakers and cardiac defibrillators. Voltage delay is undesirable because it limits the effectiveness and even the proper functioning of both the cell and the associated electrically powered device under current pulse discharge conditions.
The present invention is directed to the provision of sulfate additives in the electrolyte of an alkali metal electrochemical cell to beneficially modify the anode surface film. The sulfate additives preferably include at least one organic group containing a C(sp or sp2)-C (sp3) bond unit having the C(sp3) carbon directly connected to the xe2x80x94OSO3xe2x80x94 functional group, and are provided as a co-solvent with commonly used organic aprotic solvents. The organic sulfate additives are in a condensed phase which makes them easy to handle in electrolyte preparation. When used as a co-solvent in an activating electrolyte, the sulfate additives interact with the alkali metal anode to form an ionically conductive surface protective layer thereon. The conductive surface layer improves the discharge performance of the alkali metal electrochemical cell and minimizes or even eliminates voltage delay in the high current pulse discharge of such cells.
The object of the present invention is to improve the pulse discharge performance of an alkali metal electrochemical cell, and more particularly a primary lithium electrochemical cell, by the provision of at least one of a family of sulfate additives as a co-solvent in the cell""s activating nonaqueous electrolyte solution. Due to the high reduction potentials of the sulfate group vs. lithium, the sulfate additives can compete effectively with the other electrolyte co-solvents or the solute to react with the lithium anode. Lithium sulfate or the lithium salt of sulfate reduction products are believed to be the major reaction products. These lithium salts are believed to deposit on the anode surface to form an tonically conductive protective film thereon. As a consequence, the chemical composition and perhaps the morphology of the anode surface protective layer is changed, and this proves beneficial to the discharge characteristics of the cell. The thusly fabricated cell exhibits reduced or no appreciable voltage delay under current pulse discharge usage, which is an unexpected result.